Cytokines that have an "Interleukin" designation are those protein factors that influence immune effector cells. Cytokines designated interleukin-1 through interleukin-12 have been reported and named as an interleukin. Other known cytokines include tumor necrosis factor (TNF), granulocyte-macrophage colony stimulating factor (GM-CSF), granulocyte colony stimulating factor (G-CSF), mast cell growth factor (MGF), epidermal growth factor (EGF), platelet-derived growth factor (PDGF), nerve growth factor (NGF), erythropoietin (EPO), .gamma. interferon (.gamma.-IFN) and others.
DNAs for two different TNF receptors (Type I and Type II) have been cloned (Smith et al., Science 248:1019, 1990; and Schall et al., Cell 61:361, 1990). Both forms of TNF receptor are related to each other and belong to a family of receptors whose members include nerve growth factor receptor (Johnson et al., Cell 47:545, 1986), B cell antigen CD40 (Stamenkovic et al., EMBO J. 8:1403, 1989), T cell antigen OX40 (Mallett et al., EMBO J. 9:1063, 1990), human Fas antigen (Itoh et al., Cell 66:233, 1991) and murine 4-1BB receptor (Kwon et al., Cell. Immunol. 121:414, 1989 [Kwon et al. I] and Kwon et al., Proc. Natl. Acad. Sci. USA 86:1963, 1989 [Kwon et al. II]).
Human CD40 protein (CD40) is a peptide of 277 amino acids having a molecular weight of 30,600, and a 19 amino acid secretory signal peptide comprising predominantly hydrophobic amino acids (Stamenkovic et al.). The molecular weight (exclusive of glycosylation) of the mature human CD40 protein is 28,300. A cDNA encoding human CD40 was isolated from a cDNA library prepared from Burkitt lymphoma cell line Raji. The putative protein encoded by the CD40 cDNA contains a putative leader sequence, trans-membrane domain and a number of other features common to membrane-bound receptor proteins. CD40 has been found to be expressed on B lymphocytes, epithelial cells and some carcinoma cell lines.
A monoclonal antibody (mAb) directed against CD40 has been shown to mediate various functional effects of human B cells. These effects include: (a) homotypic adhesions (Gordon et al., J. Immunol. 140:1425, 1988 [Gordon et al. I]); (b) increased cell size (Gordon et al. I and Valle et al., Eur. J. Immunol. 19:1463, 1989); (c) proliferation of B cells activated with anti-IgM, anti-CD20 mAb, phorbol ester alone (Clark et al., Proc. Natl. Acad. Sci. USA 83:4494, 1986; and Paulie et al., J. Immunol. 142:590, 1989), or phorbol ester combined with interleukin-4 (Gordon et al., Eur. J. Immunol. 17:1535, 1987 [Gordon et al. II]; and (d) production of IgE (Jabara et al., J. Exp. Med. 172:1861, 1990; Zhang et al., J. Immunol. 146:1836, 1991) and IgM (Gascan et al., J. Immunol. 147:8, 1991) from interleukin-4 (IL-4) stimulated T-depleted cultures.
One such antibody, called mAb 89 by Banchereau et al., Clin. Immunol. Spectrum 3:8, 1991 [Banchereau et al. I], was found to induce human B cell proliferation at a relatively low antibody concentration (30 ng/ml or about 10.sup.-10 M). Proliferation lasted two to three weeks and resulted in a ten-fold expansion of the human B cell population. Optimal stimulation of the B cells occurred when CD40 surface molecule was cross-linked by IgM. Fab fragments of another anti-CD40 mAb induced only a weak proliferative response. Further, Banchereau et al., Science 251:70, 1991 [Banchereau et al. II] reported that resting human B cells entered a state of sustained proliferation when incubated with both a murine fibroblastic Ltk.sup.- cell line that was transfected with human Fc receptor and with a monoclonal antibody specific for human CD40. Banchereau et al. II found that cross-linking CD40 is necessary for clonal expansion of B cells.
CD23 is a low affinity IgE receptor that has been found to be expressed on most IgM.sup.- /IgD.sup.- mature B cells, but not T cells. CD23 has been sequenced and its sequence was described in Kikutani et al., Cell 47:657, 1986. Soluble CD23 (sCD23) was found to induce a pyrogenic reaction in rabbits and this reaction was abrogated by administration of human IgE (Ghaderi et al., Immunology 73:510, 1991). Therefore, CD23 may be an appropriate marker for soluble CD40 or CD40-L effects.
Prior to the present invention, a ligand for CD40 was unknown. Accordingly, there is a need in the art to identify and characterize a CD40 ligand (CD40-L).